The invention relates to the field of signal processing of received television signals, and in particular to channel filtering of analog or digitally modulated television signals.
Television signals are transmitted on various frequency channels. A tuner in a television receiver selects a channel based on the setting provided by the operator, then amplifies this channel. In most television receivers, the selected television signal is subsequently converted to a first intermediate frequency (IF) of about 40 MHz. The intermediate frequency signal is typically bandpass filtered by a surface wave filter. In many implementations of television receivers, the TV signal converted to the first intermediate frequency is then converted to a second intermediate frequency after subsequent, generally analog, signal processing. Such a conversion is described, for example, in DE 198 25 455 C2. For purposes of clear differentiation, this second intermediate frequency will be abbreviated to 2nd IF. The second intermediate frequency, 2nd IF, is selected such that the video carrier of the lower side channel is set at zero frequency. As a result, the side channel video carrier can be easily filtered out by an analog high-pass filter. Since unwanted mixing products of the second conversion must also be suppressed, and subsequent analog-to-digital conversion requires an anti-aliasing low-pass filter, this filter is also designed as a bandpass filter. The resultant signal, also called the 2nd IF signal, is then fed to another, preferably digital, signal processor.
Modem tuner units, so-called silicon tuners, are also able to convert the channel directly to the low IF, or use as the IF a significantly higher frequency (e.g., 1.2 GHz), and then convert to the low IF.
A multiplicity of prior art techniques effect channel filtering of an analog or digitally modulated TV signal of a channel (selected from a tuner) having a video carrier signal converted to an intermediate frequency (IF or 2nd IF), video signal(s), generally two sound carrier signals, and sound signal(s). Although these techniques have in principle proven successful, they all require special transmission-standard-dependent prefilters. Otherwise, generally not all interfering signals from adjacent channels—specifically, higher or lower side channels—are effectively suppressed or filtered out if only one prefilter is to be used. In addition, with these methods the prefilters have completely different amplitude responses relative to the prefilters required for digital television signals.
There is a need for improved channel filtering of analog or digitally modulated TV signals converted to an intermediate frequency, which suppresses interference more effectively, and uses the same standard-independent prefiltering for analog and digitally modulated television signals.